I don’t think McConnell wanted control of the Senate. Otherwise they would have bent over backwards to work a deal out with Manchin and Sienna to get them to caucus with Republicans. Either one would be more conservative than some of the so-called Republicans in the senate. An opportunity lost.
I think if Walker won in Georgia, Manchin would have switched. Once Walker lost there was no sense Manchin switching because the Dems would still have the 50-50 majority.